There is something scary but at the same time sexy about the Gorgon Queen Medusa. In Greek mythology, she was one of three sisters who had the power to turn men to stone should they gaze upon her in her altered form. Medusa's ability to turn men to stone with the evil serpents in her hair blend in marvellously for Halloween.
What makes Medusa sexy is her confidence and attitude. "Love me or suffer the consequences!" Her female form was originally depicted as hideous but has since evolved into a beautiful woman with hidden monstrosities and powers that surface when angered. Her personae has gotten sexier over the years and if you add your plus size she will be even better. Medusa met a tragic end by having her head cut off, so when you are out partying stay away from any guys going by the name Perseus!
Buy Costumes had an elite costume which is higher end but quite nice in 2x and 3x (keep an eye out for restock). There is also a Medusa, The Mythical Siren with lovely iridescent gown and snake headpiece -- speaking of headpieces, they have a few different options besides what is shown with a costume so make sure you use their search database to see them all. Costume, check. Hair, check. All you need is some sexy pumps and accentuating make-up. And maybe a few extra snake toys to hand out to your special friends.
If you are on a tight budget this would be a simple costume to make on your own. Any simple long black dress will do. Just add a few sashes around the waist for accent and a trip to the dollar store could provide you with a bucket of plastic snakes to work throughout your hair with pretty ribbons.
Check out Buy Costumes for their plus size Medusa costume selection.